MATERIALS RECOVERY FROM USED LEAD-ACID BATTERIES

Transcription

1 MATERIALS RECOVERY FROM USED LEAD-ACID BATTERIES Wiesław Frącz 1, Feliks Stachowicz 1, 1 Rzeszów University of Technology, ul. W. Pola 2, Rzeszów, Poland Abstract: There are different alternatives to the final disposition of batteries: landfill, stabilization, incineration and recycling. The number of processes for battery recycling has increased over the last few years, mainly because of the environmental impact caused by their disposal. The purpose of this paper is to review the current status of the lead-acid battery recycling technology currently use as well as methods of battery characteristics improving. Key words: recycling, used battery, environmental protection 1. INTRODUCTION The wide scale reuse and recycling of products to avoid the disposal of concentrated materials is a strategy that both intensifies materials use and reduces that disposal. This may help in achieving more suitable patterns of production and consumption for the world population that expects ever higher standards of living. Furthermore, scraped products have to be taken back and recycled by the producers at the end of their life. The fact that recycling mass produces consumer products such as white goods, refrigerators and washing machines, is not new, and sophisticated infrastructures for scrap metal have been in place for decades. An especially great effort to recycle batteries has been made in the last two decades. The material utilization process for battery manufacturing, new residues, old residues and for reprocessing of secondary materials schematically describes a battery manufacturing and recycling system. The utilization factor can be interpreted as the total utilization of a metal in battery manufacturing including collection and reprocessing of new residues, as the total efficiency of the recycling of used batteries [1]. A battery is an electrochemical device that has the ability to convert chemical energy to electrical energy. A basic battery consists of an anode, a cathode, an electrolyte, separators and the external case. The main difference between different battery systems is the materials used as an electrode and electrolytes, which determines the specific characteristics of the systems. Separators are made of polymeric materials, paper or paperboard. Electrodes and electrolytes change as a function of the different applications of batteries. Typical household-type batteries are used in consumer items such as telephones, flashlights, radios, watches as well as in automobile vehicles. In order to promote battery recycling, it is necessary to know its composition. Unfortunately, there is no relation between the size or shape of batteries and their composition. The potentially hazardous components of batteries include mercury, copper, zinc, lead, cadmium, cobalt, manganese, nickel and lithium. There are two basic types of batteries: single-use cells and rechargeable secondary cells. The use of portable rechargeable battery cells and their effects on global metal flows were assessed for the following cases [2]: (i) the base case, which reflects the situation of the global production of batteries, (ii) the global production of portable nickel-cadmium batteries, assumed to be replaced by other battery types, (iii) assessment of the projected battery market. 31

2 To decrease the impact on global metal flows arising from the use of portable batteries, the following points should be considered: (i) the development of battery technologies should aim at high energy density and long service life, (ii) metals with high natural occurrence should be used, (iii) metals from disused batteries should be recovered and regulations implemented to decrease the need for mining of virgin metals. The lead-acid batteries have been widely used as secondary sources of energy for almost 150 years. High specific energy, high-rate discharge capacity, low cost in both manufacturing and recycling and finally high energy density is the most important characteristics of this kind of batteries resulting into their growing usage. On the other hand, current characteristics of lead-acid batteries are not optimum and should be improved. In order to improve the performance of lead-acid batteries trial-and-error methods, usually based on experimental tests, have been used for many years [3]. The aim of this paper is to review the current status of the battery recycling technologies currently used and those being developed as well as some methods of lead-acid batteries characteristics improving. 2. RECOVERY OF METALS FROM LEAD-ACID BATTERIES Lead acid batteries are usually composed of two plates: one of a Pb alloy and the other of a PbO 2 base. The PbO 2 base plate is essentially composed of a Pb alloy grate in which a PbO 2 paste is impregnated. The grates are normally composed of alloys containing low amounts of Ca, Sb and Sn. The electrolyte used is a sulfuric acid solution. A polypropylene box typically contains the electrolyte and a group of six cells. The continuous development of automotive industry has increased the amount of used car batteries requiring recycling. As was mentioned earlier, over the last decades there has been a significant increase in the quantity of batteries disposed of as domestic waste. On the other hand, resource constrains of some raw materials also require recovery of metals from used batteries. These facts and the possibility of environmental impact due to improper disposal of batteries have caused establishing regulations about the disposal of such products motivating their recycling. When the fuel engine batteries are concern, recycling process is mainly oriented on recovery of lead. The recycling process of lead-acid battery starts at the point where old battery is returned to the distributor. Lead acid batteries recycling processes are very similar to the primary lead production process [4]. The main difference is in material preparation before the reduction. The sequential steps are normally the acid removal, separation of plastic case, metallic lead and paste separation, reduction, refining and casting. Acid, polypropylene and lead are recovered in the recycling process (Fig. 1). The battery scrap processing starts from crushing the batteries, and the electrolyte contained in them is being collected. The electrolyte is later subjected to separate treatment (sedimentation, absorption and filtration) to reduce the Pb content. The crushed batteries are classified into two fractions, over 3 mm and below 3 mm in size. Metallic and organic fractions are segregated by heavy liquid separation, and a paste fraction and intermediate fraction are separated from the second one. The metallic and intermediate fractions, together with an iron scrap separated before battery crushing, constitute a metalliferrous fraction containing mainly lead and paste residues. Lead containing materials (after humidity reduction) with some additives (coke, soda, Fe scrap) make rotaryrocking furnace charge. The metalliferrous and paste fraction are melted separately in temperature up of C with the addition of recycled dust. As soon as the process cycle is completed, the melt is being poured into a single ladle in which lead is separated from slag by gravity and cast into slabs. 32

3 Spent Pb-acid batteries Milling Acid Separation CaCO 3 Neutralization Fe/NaOH Paste Evaporation Filtering Solid Coal Reduction Contaminated PP Pb Matte Slag Water Refine Disposal Lead Washing Casting Water PP to recycle Pb Fig. 1. Flowchart of the pyrometallurgical recycling process of lead acid batteries 3. RECOVERY OF PLASTICS One of the products of lead acid batteries recycling process is polypropylene. Polypropylene PP has become one of the most common plastics used in the manufacturing of automotive and some industrial lead-acid battery casings. The polymer is a thermoplastic and can be heated to a melt in a matter of minutes and moulded into a variety of shape and sizes with great ease. The properties of PP are well suited for their use as battery case material, where it can withstand the harsh environment conditions under particular applications. These include a wide temperature range, chemical resistance to acid, fuel, oils and antifreeze, physical shock, good elasticity and low shrinkage to accommodate other material components such as lead post terminals. The crushed PP is separated from other components of the lead-acid battery (Fig. 1), the chips are extensively washed in order to eliminate any traces of lead-containing particles and 33

4 acid which might have collected on its surface. The washed chips are then fed into an extruder where they are melted at approximately C. The molten polymer is then pushed trough a die after which it solidifies and the extruded plastic is then pelletized. The high melting temperature of the extruder allows for complete mixing of all chips that originate from a variety of battery case types and speed up the recycling process. The use of recycled polypropylene has cost saving implications, but it does have a disadvantage, because the material starts to deteriorate after multiple processes [5]. The mechanisms for the degradation of polypropylene are usually complicated and depend mostly on the processing environment and the level of exposure experienced during the applications. The battery manufacturer requires the battery housing to have relatively high impact resistance over a wide range of temperatures and to mould effectively around protruding lead terminals. A common practice is to restrict the use of recycled polypropylene by using a mixture of virgin and recycled material in the manufacturing of the components. The waste recycling process is also required by the Environmental Protection Agency in terms of the volatile emission generated during processing and the final disposal of the material. If and when the spent recycled PP is considered as non-recyclable, it would either be disposed of as land-fill waste or incinerated. Guidelines are given for the recycling of the complete lead-acid battery, but there are no clear directives for the requirements and environmental monitoring of various heavy metals found in the plastic components themselves that have become non-recyclable [5]. 4. NEW TRENDS IN LEAD-ACID BATTERY MARKET Future vehicle applications require the development of reliable and long life batteries operating under high-rate working conditions. Some of methods for improving lead-acid battery characteristics are as follow: (i) introduction of the 36 V battery systems, (ii) new lowantimony alloy for straps, (iii) optimization of the positive anode material, (iv) valueregulated lead-acid (VRLA) batteries. In the Toyota Crown in the autumn of 2001 the production of vehicles utilizing 36 V battery systems started, which presents a huge opportunity for the lead-acid battery and lead industries. Consumer created comforts such as heated windshields, heated and cooled seats, rapid warm or cool cabins, and variety of electronic enhancements are more likely to accelerate movement of this battery [8]. The 36 V batteries are larger and contain more lead than conventional starter batteries. The increase in batteries and lead content is expected to increase the requirements for recycling capacity for the new 36 V batteries. The lead industry has the opportunity to experience a dramatic increase in lead consumption and at the same time battery companies have the opportunity to increase the return on their product. Increased loads on the batteries have resulted in greater demands for improve of recharge performance from greater depths-of-discharge. During the past over ten years, there has been visible change in the composition of the lead alloys used for grids in automotive batteries. Usually lead-antimony alloys used for the positive grids in lead-acid batteries have generally used antimony contents of 4.5 wt. % and above. Grids for automotive batteries have changed from low-antimony alloys to lead-calcium and lead-calcium-tin alloys and even lead-calciumtin-silver alloy. Tin plays a minor part in the mechanical properties of these alloys. It is used for mould fill, recovery from deep discharge of lower antimony alloys, and bonding of grips to straps. The high tin content of lead-calcium, tin-, and silver-based alloys reduce the rate of corrosion at elevated temperature. Lead-calcium alloys in general have much higher tin in contents than lead-antimony alloys used for the same product. When lead-acid batteries are recycled by conventional technology, the major alloying element to be recovered is antimony. This has resulted in the development of low tin contents for lead-antimony alloy specifica- 34

5 tions. The tin has been lost to the slag in most battery recycling processes. Where the tin can is recovered, it cannot be utilized back into lead-calcium alloys because of high impurity content of the recovered bullion. In this alloys, all the tin must be added as pure metal. When recovered in a lead-antimony alloy, the tin may be utilized to produce low-antimony high-tin alloys with unique properties for straps, bushings, and terminals [9]. Lead dioxide (PbO 2 ) is an important oxide material used extensively as anode material in batteries and fuel cells. It is well known that PbO 2 exhibits excellent chemical stability, high conductivity, large over potential and chemical inertness for electrolysis in an acid medium. Recently PbO 2 anodes have sparked a worldwide interest because of their structural, morphological, optical, and mechanical properties and their potential application in waste water treatment, ozone generation, analytical sensors, electro-winning metals and battery electrodes. The properties of the PbO 2 films were found to be influenced by the bath temperature and solution ph. Films deposited at higher bath temperatures and low solution ph values are rich in lead content and low oxygen content. Future vehicle applications require the development of reliable and long-life batteries operating under high-rate partial-state-of-charge conditions. Depending on the power requirements and vehicle hybridization degree, several drivetrain and powernet architectures, with nominal voltages ranging from 14 V to over 600 V in hybrid buses, have been proposed. At present energy-systems for hybrid-electric vehicles applications include valve-regulated leadacid (VRLA), nickel-metal-hydride, rechargeable lithium batteries and supercapacitor [7]. It is obvious that the VRLA battery has the great advantages in terms of low initial cost, wellestablished efficiency compared to other competitive technologies at their current stage of development. Nevertheless, the running cost of the VRLA battery is expensive because of the short service life. Lead-acid batteries are nowadays extensively used in automotive applications for engine starting, lightening and ignition. The VRLA lead-acid batteries are also available for vehicles which demand high power linked to a higher capacity throughput due to the higher vehicle energy consumption demands. Moreover, VRLA batteries with spiral wound design [10] provide outstanding performance in terms of power capability and life under different cycling conditions when compared to prismatic design. To replace the complex and high cost supercapacitor lead-acid battery system an advance VRLA Ultrabattery has been developed [6, 10]. A lead-acid cell compromises one leaddioxide positive plate and one sponge lead negative plate (Fig. 2). On the other hand, an asymmetric supercapacitor composes of one lead-dioxide positive plate and one carbon-based negative plate. Since the positive plates in the lead-acid cell and the asymmetric supercapacitor have a common composition, these two devices can be integrated into one unit cell by internally connecting the capacitor electrode and the lead negative plate in parallel. The Ultrabatteries show at least four times longer in cycling performance than the control VRLA lead-acid cells and batteries. 5. CONCLUSION Environmental integrated production and recycling planning is of great importance for the competitive position of production enterprises. The minimization and recycling of byproducts and industrial waste are becoming more and more important goals for planning and controlling industrial production systems. Transport is one of the largest sources of human-induced greenhouse gas emission and fossil-fuels consumption. Thus, the ideal future transport should be directed towards the use of zero-emission vehicles on a life cycle basis. New generation of lead-acid batteries and modern recycling technologies seemed to be important source for environment protection. 35

6 6. REFERENCES Fig. 2. Schematic diagram showing the configuration of Ultrabattery [1] Anderson B.A., Rade I.: Metal source constrains for electric-vehicle batteries, Transportation Research Part D 6 (2001) [2] Bernandes A.M., Espinosa D.C.R., Tenório J.A.S.: Recycling of batteries: a review of current processes and technologies, J. Power Sources, 130 (2004) [3] Esfahanian V., Torabi F., Mosahebi A.: An innovative computational algorithm for simulation of lead-acid batteries, J. Power Sources, 176 (2008) [4] Espinosa D.C.R., Bernandes A.M., Tenório J.A.S.: An overview on the current processes for the recycling of batteries, J. Power Sources, 135 (2004) [5] Ferg E.E., Rust N.: The effect of PB and other elements found in recycled polypropylene on the manufacturing of lead-acid battery cases, Polymer Testing, 26 (2007) [6] International Patent Application No. PCT/AU 2004/ [7] Lam L.T., Louey R., Haigh N.P., Lim O.V., Vella D.G., Phyland C.G., Vu L.H., Furukawa J., Takada T., Minma D., Kano T.: VRLA Ultrabattery for high-rate partial-state-of-charge operation, J. Power Sources 174 (2007) [8] Prengaman R.D.: The impact of new 36 V lead-acid battery systems on lead consumption, J. Power Sources 116 (2003) [9] Prengaman R.D.: New low-antimony alloy for straps cycling service in lead-acid batteries, J. Power Sources 158 (2006) [10] Soria M.L., Trinidad F., Lacadena J.M., Valenciano J., Arce G.: Spiral wound valve-regulated lead-acid batteries for hybrid vehicles, J. Power Sources 174 (2007)